1. Field of the Invention
The present invention relates to a method for heat treating materials at high temperatures, and a furnace bottom construction for high temperature furnaces.
More specifically, the invention relates to a method and to the construction of a furnace chamber floor and furnace bottom insulation that makes possible the heat treatment of a material which is very likely to react with the material from which the furnace chamber floor is made at the heat treatment temperatures concerned.
2. Description of the Related Art
In some industrial applications, there is a need to heat-treat material at very high temperatures. At times, it is necessary to heat-treat material at temperatures close to their melting points.
The diffusion coefficient of some substances is very high at such high temperatures. This means that atoms from an object A that consists to 100% of the material A that is being heat-treated will diffuse into underlying material B that consists to 100% of the material B, which may be a furnace bottom or a crucible that forms the furnace bottom and that is intended to receive the material to be heat-treated. The material B will, of course, also diffuse into the material A. The maximum temperature that can be used in the heat treatment process is limited by the temperature at which said materials first begin to form a smelt.
Monocrystalline and polycrystalline oxidic material such as aluminium oxide and other aluminium-oxide based materials, such as YAG, i.e. 3Y.sub.2 O.sub.3 * 5Al.sub.2 O.sub.3 or TiO.sub.2, are examples of materials that it is desirable to heat-treat at temperatures very close to their melting points.
A common feature of these materials is that they have a very high melting point when in very pure states. One furnace alternative for heat-treating such materials are furnaces that are heated by means of electrical resistance elements of zirconium dioxide, these furnaces also being the only alternative when the heat treatment shall be carried out in an oxidizing atmosphere. Furnaces based on zirconium-dioxide elements are described in U.S. Pat. Nos. 4,041,236 and 3,440,322, and also in Swedish Patent Specification 9502475-8. When the furnaces are constructed as box furnaces or elevator furnaces, the floor of the furnace chamber will be made of the same material as the walls and ceiling or roof of the chamber, which may be zirconium dioxide, magnesium oxide, calcium oxide or some other oxidic material, or a combination of oxidic materials.
With the view of highlighting the problems that exist, it is assumed that aluminium oxide shall be heat-treated at 2000.degree. C. In this regard, Al.sub.2 O.sub.3 --MgO forms a eutectic at 1995.degree. C., CaO--Al.sub.2 O.sub.3 forms a eutectic at about 1600.degree. C., and ZrO.sub.2 --Al.sub.2 O.sub.3 forms a eutectic at 1845.degree. C. None of these material combinations can therefore be used to heat-treat aluminium oxide at 2000.degree. C.
The present invention solves this problem and enables heat treatment to be carried out at temperatures higher than the temperature at which a eutectic is formed between the material to be heat-treated and the material from which the floor of the furnace chamber is made.